1. Field of the Invention
The invention relates to a process for desulfurization of iron melts outside a blast furnace. Iron melts here are taken to mean pig iron and cast iron melts.
2. Background Information
The desulfurization of pig iron, outside a blast furnace, in a torpedo or open ladle is now part of the state of the art. Mixtures based on calcium carbide have prevailed as the preferred desulfurization agent, since these cause rapid desulfurization of a pig iron, along with high economy, and lead to low final sulfur contents. A particularly preferred agent is represented by a mixture comprising 20 to 90% by weight of industrial calcium carbide and calcium carbonate, preferably in precipitated form, and 2 to 20% by weight of carbon distributed therein, the fine-grain calcium carbonate/carbon mixture being known under the name "diamide lime" (German Pat. No. 1,758,250).
German Auslegeschrift No. 2,531,047 has disclosed a process for desulfurizing pig iron, wherein a mixture of calcium carbide, calcium cyanamide or lime containing a proportion of 0.5 to 3.5% by weight of aluminum or magnesium powder, relative to the calcium compounds, is used as the desulfurizing agent.
In U.S. Pat. No. 3,998,625, a desulfurizing agent consisting of a combination of lime and further constituents with magnesium is described, and the use of lime with a carbonaceous material and a non-oxidizing carrier gas is recommended in U.S. Pat. No. 4,266,969.
The disadvantage of the known agents are the large quantities of slag which are obtained and which lead to undesirable deposits and incrustations, particularly in the torpedo ladles and also in open ladles, and in addition enclose considerable quantities of iron, which causes considerable losses of iron.
It has also already been proposed that, in place of calcium carbonate, an additive be added to the calcium carbide which generates hydrogen at the temperature of the iron melt (German Pat. No. 2,252,796). However, such a desulfurization agent has not proven itself in practice, since the generation of hydrogen obviously does not occur in such a fashion such that an adequate dispersion of the calcium carbide in the iron melt could be effected.
It has been disclosed that calcium carbide can be employed during the treatment of cast iron melts as a desulfurization agent together with carbon, for example, in the form of pitch coke, animal charcoal or leather charcoal, but the types of coal proposed contain virtually no volatile components (see the state of the art indicated in German Pat. No. 1,758,250).
An object of the parent application is to provide a desulfurization agent, based on calcium carbide, which, on the one hand, does not introduce further slag-forming components into the iron melt and, on the other hand, evolves an amount of gas immediately after entry into the iron melt, which is adequate for the dispersion of the calcium carbide. Furthermore, advantageous consumption values, short treatment times and low final sulfur contents should be achieved using such desulfurization agent.
This object is achieved by a fine-grain agent, which is injected into the iron melt in fluidized form by means of a gas, which agent comprises a mixture of industrial calcium carbide and a dried coal which contains at least 15% by weight of volatile components and magnesium.